This invention related to a brake booster for an automotive vehicle utilizing the differential of pressure between a vacuum and atmospheric pressure comprising a vacuum casing with at least one movable wall arranged therein, wherein the movable wall divides the vacuum casing into a vacuum chamber and a working chamber and acts upon a push rod guided in an axial direction of the brake booster, and wherein at least two bars are provided which are connected to the transverse end walls of the vacuum casing, extend essentially parallel to the push rod and penetrate, and are sealed relative to, the movable wall.
Brake boosters of the aforementioned type for automotive vehicles, wherein the differential of pressure between a vacuum and atmospheric pressure is used for the generation of an additional force are known, for example, from German Pat. No. DE-AS 2,345,314. The brake booster comprises a vacuum casing, a wall movably arranged therein, and a control valve. The vacuum casing is of tubular design, its two transverse end walls serving as fastenings. The brake booster conventionally has its one transverse end wall, where the control valve is provided, fastened to the vehicle's splashboard, while the master brake cylinder is fastened to the other transverse end wall of the brake booster. The fastening means are generally bolts arranged in the end walls of the brake booster. A movable wall divides the interior of the vacuum casing into a vacuum chamber in which a constant vacuum prevails and a working chamber in which the pressure is variable dependent upon the control valve. Like the pedal force acting on a push rod via the control valve, the force acting on the movable wall in the presence of a pressure differential is transmitted to the master brake cylinder via the push rod axially disposed in the vacuum casing.
The vacuum casing is conventionally of sheet steel. With the control valve inactivated, the same vacuum will prevail in the vacuum chamber and in the working chamber. As a result of the pressure differential between the atmospheric pressure prevailing outside the vacuum casing and the vacuum, forces will be produced causing a deformation of the vacuum casing. Since there must be a limit to such deformation, the casing parts must be of correspondingly strong design. This is not economic because of increased material costs, and also, the weight of the brake booster would be above a specific minimum value. Thus, the car manufacturers' requirements for lowest possible weight of brake systems while at the same time ensuring the safety and operability of the brake boosters could not be satisfied.
In addition to the forces caused by the pressure differential, further forces transmitted via the push rod occur on brake actuation, i.e., the brake pedal force and the booster force generated by the brake booster. The sum of the forces act on the piston of the master cylinder, generating in the master cylinder a reaction force of the same magnitude, but acting in the opposite direction. This reaction force extends from the master cylinder via the sheet steel parts of the vacuum casing to the splashboard to which the pedal assembly is conventionally fastened. Due to this reaction force, the vacuum casing will be extended on brake actuation in an axial direction, causing displacement of the master cylinder and consequently an increased lost travel of the brake pedal. Because of the brake lever transmission ratio, the extensions of the lower casing shell will be multiplied when transmitted to the brake pedal.
In U.S. copending application Ser. No. 49,706, filed June 18, 1979, assigned to the same assignee as the present application and whose disclosure is incorporated herein by reference at least two bars are connected to the transverse end walls of the vacuum casing, extend essentially parallel to the push rod and penetrate the movable wall which is slidably arranged on the bars by means of slide seals.